(1) Field of the Invention
The present application relates to a thermistor, and more particularly to an over-current protection device.
(2) Description of the Related Art
Over-current protection devices are used for protecting circuitries from damages resulted from over-heat or over-current. An over-current protection device usually contains two electrodes and a resistive material disposed therebetween. The resistive material has positive temperature coefficient (PTC) characteristic that the resistance thereof remains extremely low at room temperature and instantaneously increases to thousand times when the temperature reaches a critical temperature or the circuit has over-current, so as to suppress over-current and protect the cell or the circuit device. When the material gets back to the room temperature or over-current no longer exists, the over-current protection device returns to be of low resistance and as a consequence the circuitry can operate normally. In view of the reusable property, the PTC over-current protection devices can replace traditional fuses, and have been widely applied to high density circuits.
With lightweight and compact trends, electronic apparatuses are getting smaller. For a cell phone, a number of components have to be integrated into a limited space, in which an over-current protection device is usually secured to a protective circuit module (PCM) and its external lead will occupy a certain space. Therefore, it is desirable to have a thin-type protection device that does not take up much room. When the device is downsizing to form factor 0201, it is a great challenge on how to decrease the thickness of the protection device for surface-mount applications.
According to specification of 0201, a device has a length of 0.6±0.03 mm, a width of 0.3±0.03 mm and a thickness of 0.25±0.03 mm. In manufacturing, the length and width are doable, but the thickness is too thin to be achieved. Nowadays, the resistive material substrate of carbon black system can be at most pressed to 0.2 mm in thickness, and the resistive material substrate of ceramic filler system can obtain a thickness of 0.2-0.23 mm. If the resistive substrate is further engaged with insulating (prepreg) layers and internal and external circuits (electrodes) to form an over-current protection device as shown in U.S. Pat. No. 6,377,467, not only is the thickness out of specification but also the thickness may be equal to or larger than width. As a result, the devices may topple when they are subjected to packaging or other processes afterwards.
Moreover, the internal-and-external circuit design on a small size device sometimes has misalignment between the internal circuit and external circuit, and therefore the production yield will be negatively impacted.
U.S. Pat. No. 8,044,763 disclosed the use of low resistivity materials such as metal powders or metal carbides to make surface mountable devices (SMD), which is a breakthrough to the limitation of using carbon black as conductive fillers and significantly increase hold current per area to 0.16 A/mm2, or even up to 1 A/mm2. The hold current indicates the largest current of the device before trip. However, the mobile apparatuses had greatly advanced. It is demanded that the device be of smaller volume and more powerful, and thus more operating current is needed. Therefore, the upper limit of 1 A/mm2 would be unsatisfactory, and larger hold current per effective PTC area is desired for a protection device with smaller area and larger operating current.
Therefore, it is a great challenge to make a small device with large hold current, and preferably the device structure can be simplified to decrease the process steps and manufacturing cost.